1. Field of the Invention
This invention relates to semiconductor structures, and more particularly, to forming a pattern of lines of epitaxial silicide embedded in a silicon layer or substrate.
2. Background Art
To construct a 3-dimensional structure within a silicon wafer, the essential step is to be able to build a conducting line within or below the surface of the silicon wafer. The line is required to have epitaxial relationship to the surrounding silicon, meaning on its four sides if the line has a rectangular cross-section. Using current techniques, it is easy to in-lay a non-epitaxial line in silicon, but it prevents us from building on the surface of the non-epitaxial line a 3-dimensional structure without destroying the single crystal nature of the silicon, which is crucial from the point of view of device application. Since it is difficult or nearly impossible to grow epitaxial layers above lines, metallic cobalt silicide and nickel silicide thin films have been studied as a component of an epitaxial superlattice structure on silicon, such as Si/CoSi.sub.2 /Si/CoSi.sub.2. However, the superlattice is not truly a 3-dimensional structure because the silicide is formed as a layer and not as a line or pattern of lines.
In a publication by A. Ishizaka et al. entitled "Formation of Embedded Monocrystalline NiSi.sub.2 Grid Layers in Silicon by M.B.E.", Jpn. J. Appl. Phys. Part. 2, Vol. 23, No. 7, pp. 499-501, July 1984, overgrowth of monocrystalline silicon films had been formed by the molecular beam epitaxy technique on patterned NiSi.sub.2 layer which were epitaxially grown on Si (111) substrates. The embedded NiSi.sub.2 grid layer can form a buried electrode in a transistor and in other novel devices.
In Japanese patent document JP6021558, published Feb. 2, 1985, entitled "Bi-Polar Type Semiconductor Integrated Circuit Device", a layer of cobalt is deposited over silicon, followed by a heat treatment to form cobalt silicide. The remaining cobalt was subsequently etched. Then an N type epitaxial layer was selectively formed over the cobalt silicide layer. Subsequently, a base layer and an intermediate layer are formed resulting in a bipolar transistor having a low resistant buried layer made of a silicide metallic film.
In U.S. Pat. No. 4,816,421 which issued on Mar. 28, 1989 to R. C. Dynes et al., a buried heteroepitaxial structure termed "mesotaxy" is formed by implantation of at least one chemical species such as Co, Ni, Cr, Y or Mg into a single crystal body such as Si or Ge such that a buried layer rich in the implanted species is formed and heat treated such that a buried stoichiometric compound layer such as CoSi.sub.2 is formed which is epitaxial with the Si matrix, has high conductivity and is of good crystalline quality.
In U.S. Pat. No. 4,554,045 which issued on Nov. 19, 1985 to J. C. Bean et al., a silicon substrate is shown in FIG. 3 covered with a silicide layer, a single crystal silicon layer and another silicide layer, each with appropriate electrical contacts to form, for example, Schottky diodes, back-to-back diodes, a pnpn structure or a silicon controlled rectifier. The silicide layers are composed of nickel silicide or cobalt silicide.